1. Field of the Invention
The invention pertains generally to the manufacture of glass sheets, and, more particularly, to glass sheets formed from an overflow process.
2. Description of Related Art
The glass that is used for semiconductor powered display applications, particularly for TFT/LCD display devices that are widely used for computer displays, must have very high surface quality to allow the successful application of semiconductor type material. Sheet glass made using the apparatus of U.S. Pat. No. 3,338,696, assigned to Corning, Inc., makes the highest quality glass as formed and does not require post-processing. That patent makes glass by a manufacturing process termed “The Overflow Process”. Glass made using other processes requires grinding and/or polishing and thus does not have as fine a surface finish. The glass sheet must also conform to stringent thickness variation and warp specifications. The fine surface finish is formed from virgin glass primarily from the center of the glass stream. This glass has not been in contact with foreign surfaces since the stirring operation.
The teachings of U.S. Pat. No. 3,338,696 are still the state of the art as practiced today; however, the apparatus has limitations.
A drawback of the apparatus of “The Overflow Process” is that, even though it makes excellent glass during stable operating conditions, the control of process temperature and flow is limited by simplistic control technology, thus allowing unintended process transients which cause glass defects. Defects caused by the flow and temperature transients eventually subside when the process stabilizes; however, there is a period of time where the quality of the glass sheet is substandard. Flow and temperature transients also cause variations in the sheet thickness. Therefore, there is a need in the art for measurement technology that facilitates more precise control of the flow rate of the glass entering the forming apparatus.
Another drawback of the apparatus of “The Overflow Process” is that the forming apparatus deforms during a manufacturing campaign in a manner such that the glass sheet no longer meets the thickness specification. This is a primary cause for premature termination of the production run. Therefore, there is a need in the art for technology that corrects for the apparatus deformation.
Another drawback of the apparatus of “The Overflow Process” is that the production rate of the present forming apparatus design is limited by the use of a single step process to both distribute the glass and cool the glass prior to drawing the sheet. The limited production rate makes the cost of the glass sheet that is produced less competitive in the market.